Method of electrolytic production of oxygen difluoride



United States Patent 3 414,495 METHOD OF ELECTROLYTIC PRODUCTION OF OXYGEN DIFLUORIDE John A. Donohue, Chicago, 11]., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana Filed Feb. 24, 1966, Ser. No. 529,718 7 Claims. (Cl. 204-401) This invention relates to a method of electrolysis and more particularly to an improved method for the electrosynthesis of oxygen difluoride from wet hydrogen fluoride. The electrolytic production of oxygen difluoride (GE) is typically carried out by the continuous passing of a current through a liquid electrolyte positioned in an electrolytic cell consisting essentially of hydrogen fluoride, alkali metal fluoride, and water, and withdrawing gases containing oxygen difluoride from the anode of the cell. The yield of oxygen difluoride, however, decreases with time. That is, while yield may be relatively high at first, it is found that as the cell continually generates yield will begin to decrease.

In addition, even though previous work in the electrolytic production of 0P has shown that yield may be increased by maintaining the water content and the alkali metal fluoride content of the electrolyte within particular ranges, the yield while initially higher, will, nevertheless, begin to decrease in time. For example, it is not uncommon for yields of 0P to fall from 35% to 20% (yield expressed as a percent of current passed) over a five-hour period. Of course, in a commercial installation for the electrolytic production of OR, this is highly undesirable. There is, therefore, a need for a method which would provide for increased yields of products such as 0P by electrochemical methods.

A process of electrolysis of wet hydrogen fluoride has now been discovered which offers the advantages of increased product yield and which enables the yield from the electrolytic production of compounds such as OF to be maintained at these increased levels.

Briefly stated, this inventon comprises the steps of passing an electrical current through a liquid electrolyte positioned in an electrolytic cell consisting essentially of hydrogen fluoride, alkali metal fluoride, and water, periodically interrupting the flow of the current for a time and withdrawing from the cell gases predominately containing oxygen difluoride. The electrolyte should be maintained at a temperature less than 250 C. and consist essentially of hydrogen fluoride, alkali metal fluoride in the range of about .2 to 5. mole percent, and water in the range of about .2 to 2.0 mole percent.

While the exact reason for the increased yield of 0P is not clear and while applicant does not wish to be bound by any one particular theory, it is believed that the key may be found in the decomposition of Water as the primary electrochemical reaction. Support for this theory has been found by observing the effects of operation near dryness, alkali metal fluoride concentration, and anode Weight losses.

In the present example an anode of nickel is used. The anode while not active in the sense of a growing film of MP is certainly not passive. Current increase during the induction period points to changes in the semi-conducting properties of the anode film. Changes of yield with time during continuous operation would seem to indicate continuously changing surface properties. Interrupted operation, on the other hand, appears to result because of the steady yields in an equilibrium at the anode surface.

The source of fluorine for 0P appears to be the NiF in the anode film. Any source (F or HP) in the electrolyte would always be as readily available as water and no decrease in 0P yield with time would be expected.

However, the coulometry is such that all the fluorine in the thin NiF film would be rapidly depleted. Since 0P yields do not drop to zero in a matter of seconds, it appears that the surface has some rapid means of obtaining fluorine from the electrolyte. The chemical decomposition of an unstable oxide to reform NiF is an obvious device. Interrupted operation is then necessary to maintain OF yield whenever oxide formation is faster than its decomposition.

The illustration, accompanying Example 1, will more clearly show the effect interrupted operation has on OF yields.

A specific embodiment of the process of this invention as employed in the electrolytic production of OF from wet hydrogen fluoride is as follows:

The electrolytic method of the invention can be carried out in any electrolytic cell wherein a liquid electrolyte can be positioned in the cell and an electric current passed therethrough; more commonly electrodes are immersed in the liquid electrolyte and provisions are made for maintaining the liquid electrolyte at the desired temperature of operation. Electrolytic cells suitable for the carrying out of the method of the invention include those now used for the electrolytic production of fluorine except that the carbon anode needs to be replace-d by a metal anode. It is to be understood that the cells may be modified 'by anyone of ordinary skill with materials of construction suitable for use with the particular electrolytes used herein. It has been found that particularly suitable materials of construction for the electrodes are: anode formed of nickel and cathode formed of iron, specifically, black iron.

When an electric current is passed through a liquid electrolyte consisting of hydrogen fluoride, dissolved inorganic fiuoride and about 0.052.0 mole percent of water, gases are produced. These gases include hydrogen, molecular oxygen, oxygen difluoride (CR and ozone (0 The hydrogen is produced at the cathode, the other gases being anode products. Occasionally insignificant amounts of fluorine (F are also produced at the anode.

The presence of hydrogen in the product gases may be drastically reduced or essentially eliminated by bathing the cathode with oxygen which reacts with the hydrogen produced at the cathode to form water. This provides make-up water in the electrolyte and increases safety. An especially convenient method of doing this is to form the cathode of porous metal and pass the oxygen through it. By using a porous cathode the oxygen is highly dispersed and a large cathode surface is provided so that the reaction of oxygen with the hydrogen is enhanced. Other suitable methods for providing oxygen at or near the cathode for reaction with the hydrogen can be devised by those skilled in the art. The water produced dissolves in the electrolyte. Under most operating conditions using oxygen at the cathode the water concentration in the electrolyte will tend to build up, because it is formed at a higher rate than it is consumed, so that it will be necessary to remove water in order to maintain the desired concentration. A convenient source of oxygen is that produced at the anode of the cell and separated from the other anode products.

It is to be understood that hydrogen fluoride (HF) as used herein may include regular commercial grade acid as well as high purity hydrogen fluoride itself. When utilizing commercial grade acid, the water content thereof will be calculated as part of the total desired water content of the particular liquid electrolyte present in the electrolyte cell.

In addition to hydrogen fluoride and water the electrolyte used in the method of the invention contains HF- soluble inorganic fluoride. The inorganic fluoride must have a sufficient solubility in liquid hydrogen fluoride to pass the electrolytic current in a significant amount. When the current is passed through an electrolyte consisting only of hydrogen fluoride and water, ozone is normally produced in larger amounts than oxygen difluoride is produced. However, when an inorganic fluoride is present, dissolved in the electrolyte, the oxygen difluoride is produced in a greater amount than is ozone, in the ranges of electrolyte composition of most interest to commercial operation. At certain combinations of hydrogen fluoride, dissolved inorganic fluoride and water the production of oxygen difluoride and ozone is more or less equal; in general, the production of oxygen difluoride is substantially greater than the production of ozone.

The inorganic fluorides are desirably metal fluorides where the metal component is positioned above hydrogen in the electromotive series of metals. A listing of these is presented at page 666, Langs Handbook of Chemistry (1934). The alkali metal fluorides are particularly suitable inorganic fluorides for use in the method of the invention. Potassium fluoride, sodium fluoride and lithium fluoride are preferred.

Sufficient inorganic fluoride must be present in the electrolyte to shift the production of oxygen difluoride toward the predominant position relative to ozone. More than this amount may be used. The maximum amount of inorganic fluoride is determined in part by the water content of the electrolyte and the temperature of operation for the particular cell, and keeping the production of fluorine to either none or an insignificant amount. When it is desired to maximize the production of P relative to ozone, the alkali metal fluoride present in the electrolyte should be in the order of from about 0.2 to 5.0 mole percent of the total electrolyte. The preferred range is between about .5 to 1.0 mole percent.

Some water must be present in the electrolyte in order to keep the production of fluorine at least at an insignificant amount; normally no fluorine is produced. Thus the amount of water present in the electrolyte is related to the desired production of only insignificant amounts of fluorine or none at all, and the desired production of ozone along with the oxygen difluoride. The water in the electrolyte is a convenient source of oxygen for the production of ozone and oxygen difluoride. Broadly, the water content of the electrolyte is about 0.1 to 2.0 mole percent of the total electrolyte. Usually the water content of the electrolyte is about 0.2 to 1.0 mole percent. When it is desired to maximize production of oxygen difluoride relative to production of ozone, using the preferred range of alkali metal fluoride in the electrolyte, the preferred water content of the electrolyte is about .3 to .5 mole percent.

It is to be understood that the electrolyte used in the method of the invention is used as the basis of determining the mole percentages set forth above and that the hydrogen fluoride portion makes up essentially the remaining amount after the inorganic fluoride water contents have been specified.

The electrolyte must be in the liquid state and sufficient pressure must be maintained on the cell to keep the electrolyte in the liquid state at the particular temperature of operation.

The electrolytic cell is operated at any temperature which will permit the production of oxygen difluoride and produce no or only insignificant amounts of fluorine (P In general, the cell is operated at a temperature of not more than about 250 C. and desirably at a temperature of not more than about 100 C. More commonly, the cell is operated at a temperature from about -30 C. to +50 C.

The product of the electrolysis comprises a mixture of hydrogen, oxygen, oxygen difluoride and ozone. (Fluorine is either not present or produced in an insignificant amount.) The hydrogen may be readily separated from the other gaseous products by condensing these three. The

oxygen and ozone may be removed by low temperature distillation from the oxygen difluoride. It has been discovered that silica gel adsorbs ozone in preference to oxygen difluoride and essentially pure oxygen difluoride may be recovered as efliuent from the silica gel adsorption zone.

The terms, length and frequency of current interruption as used herein should be understood to show the following meanings. The length of interruption is the duration of the current interruption. The frequency of interruption is how often each current interruption is made. For example, a length of interruption might be one second at a frequency of every two minutes. Or the length of interruption might be two seconds at a frequency of every five minutes. In addition the length and frequency of interruption may be varied with respect to each other. That is, the length of one interruption might be one second and the next two seconds and so forth. This varied length of interruption might take place at different frequencies. That is, the first interruption might have a length of one second, the second a length of one second two minutes later, the third at length of two seconds five minutes later, and so on. However, for best results, the length and frequency of interruption should occur at times sufficient to allow the cell to stabilize. This will depend somewhat on the characteristics of the individual cell. Optimumly, the length and frequency of current interruption as employed in the present invention should occur for a'length of time between about 0.1 second to about ten seconds and at a frequency of about .1 minute to about ten minutes. Preferably the length and frequency of interruption in the above embodiment should be about a one second interruption every two minutes. However, these factors will depend somewhat on the range of reaction parameters of the particular electrolyte; the interruption rate is the prime factor and it can be maximized for any given set of secondary variables (i.e., water concentration, alkali metal fluoride concentration, voltage, temperature, etc.). The above ranges have produced satisfactory results in the particular application set forth. As explained above, the interruption length and frequency may be regularly or irregularly varied with respect to each other.

The following examples illustrate the invention.

A suitable cell for use in this work could be a cell composed of a Kel-F cup covered by a stainless steel cap. The use of Kel-F allows the contents of the cell to be observed during the electrolysis. The cup was ap proximately 2 in diameter and 4" high.

No cell diaphragm was used since earlier work showed that separation of the anode and cathode compartments was unnecessary. Two nickel anodes and three iron cathodes, 3" x 1 /8", made up the electrode pack. Teflon spacers CA") were used, and the pack was held together by steel bolts insulated with Teflon sleeves. The electrolyte was cooled by an external ice bath and a copper cooling tube placed in the electrolyte. Acetone from a Dry Ice bath (78 C.) was circulated through this tube to regulate the electrolyte temperature. The coolant flow was regulated by a temperature controller connected to a thermocouple in the electrolyte. Both the thermocouple well and the coolant coil were coated with Kel-F wax to prevent corrosion.

A suitable interrupter for use with the cell may be made from a relay and a timer such as a mercury Wetted relay and the Flexopulse timer.

Reference in the accompanying figure to a sparger refers to the passing of helium flush gas into the electrolyte below the electrodes. This increases stirring and tends to break up gas films on the electrodes.

EXAMPLE 1 The figure illustrates the effects of interrupted opera tion versus continuous operation on the electrolytic production of 0P from wet hydrogen fluoride. The reaction parameters are set forth in the figure. For the first four hours the cell Was operated continuously and 0P yield did not exceed When interrupted operation was begun, the 0P yield rose to more than 45% and the current density also increased considerably. Although yield increases more slowly with interrupted operation, it can be seen that once it peaks, it is far more static and yields are much higher than with continuous operation.

EXAMPLE 2 This example is included to illustrate the reproducibility and stability of interrupted operation versus continuous operation.

The cell conditions were as follows:

KF=1.0 mole percent.

Cell potential=6.0 v.

H O range=1.43 to 1.58 mole percent with sparger. Electrolyte temperature=8 to 13 C.

Three runs were made employing interrupted operation. The length of interruptions was one second at a frequency of 120 seconds in each case. The results, in contrast to a continuous run under the same conditions, are as follows:

Yield 1 OF2 Yield 1 OF? Time (Min.) 4 interrupted continuous operation runs operation 1 Yield expressed as percent of current.

While the invention has been described and illustrated with considerable particularity in a preferred embodiment, it is to be understood that the invention is not limited to the embodiment which has been illustrated and described, since variations and modifications may be made therein without departing from the spirit and scope of the invention.

Having thus described the invention, what is claimed 1s:

1. A process for producing oxygen difluoride from hydrogen fluoride and water which process comprises passing an electrical current through a liquid electrolyte positioned in an electrolytic cell, said liquid electrolyte being maintained at a temperature of less than 250 C. and consisting essentially of hydrogen fluoride, alkali metal fluoride within the range of about .2 to 5.0 mole percent and water within the range of about .1 to 2.0

' mole percent, periodically interrupting the flow of said current for a time and withdrawing from said cell gases containing oxygen difluoride, said interrupting causing greater yield of oxygen difluoride, based on current passed, than Without said interrupting.

2. The process of claim 1 wherein the flow of said current is interrupted in intervals for a period of time between about .1 second to about 10 seconds.

3. The process of claim 1 wherein periodically interrupting the flow of said current occurs in intervals be tween about .1 minute to about 10 minutes.

4. The process of claim 1 wherein the flow of said current is periodically interrupted in intervals between about .1 minute to 10 minutes, the duration of each interruption being between about .1 second to about 10 seconds.

5. The process of claim 1 wherein periodically interrupting the flow of said current occurs in irregular intervals between about .1 minute to 10 minutes and for a duration of time between about .1 second to about 10 seconds.

6. The process of claim 1 wherein periodically interrupting the flow of said current occurs in intervals between about .1 minute to about 10 minutes and for periods of time of irregular duration between about .1 second to about 10 seconds.

7. The process of claim 1 wherein periodically interrupting the flow of said current occurs in irregular intervals between about .1 minute to about 10 minutes and for periods of time of irregular duration between about .1 second to about 10' seconds.

References Cited UNITED STATES PATENTS 2,519,983 8/1950 Simons 204-59 2,806,817 9/1957 Wolfe 20459 3,367,744 2/1968 Brown et a1. 23205 HOWARD S. WILLIAMS, Primary Examiner. D. R. JORDAN, Assistant Examiner. 

1. A PROCESS FOR PRODUCING OXYGEN DIFLUROIDE FROM HYDROGEN FLUORIDE AND WATER WHICH PROCESS COMPRISES PASSING AN ELECTRICAL CURRENT THROUGH A LIQUID ELECTROLYTE POSITIONED IN AN LECTROLYTIC CELL, SAID LIQUID ELECTROLYTE BEING MAINTAINED AT A TEMPERATURE OF LESS THAN 250* C. AND CONSISTING ESSENTIALLY OF HYDROGEN FLUORIDE, ALKALI METAL FLUORIDE WITHIN THE RANGE OF ABOUT .2 TO 5.0 MOLE PERCENT AND WATER WITHIN THE RANGE OF ABOUT .1 TO 2.0 